1. Field of the Invention
The present invention relates to a hydraulic pressure supply system for a variable torque transfer of a four-wheel drive vehicle, and specifically, to a hydraulic pressure supply system that can continuously control a value of hydraulic pressure fed to a variable torque transfer clutch of a four-wheel drive vehicle in response to a control command generated from a control unit, to variably adjust a distribution ratio of driving torque between front and rear road wheels.
2. Description of the Prior Art
Recently, there have been proposed and developed various four-wheel drive vehicles equipped with a variable torque transfer which can continuously adjust a distribution ratio of driving torque between front and rear road wheels by varying an engaging force of the transfer clutch. The four-wheel drive vehicle with a variable torque transfer also includes a hydraulic pressure supply system for the transfer, so as to variably adjust a clutch pressure fed to a transfer clutch. One such transfer clutch pressure supply system has been disclosed in Japanese Patent Provisional Publication (Tokkai Heisei) No. 2-68225, assigned by the same assignee as the present application, entitled "CONTROL DEVICE FOR ALLOCATION OF DRIVING FORCE FOR FOUR-WHEEL DRIVE VEHICLE". Such a prior art hydraulic pressure supply system for a variable torque transfer is often mounted in high-grade four-wheel drive vehicles, in order to perform a total control consisting of a front-and-rear wheel speed difference dependent traction control, a four wheel anti-skid brake control, and the like, respectively combined with a variable driving torque control of the transfer clutch. Referring now to FIG. 1, there is shown a four-wheel drive vehicle with a conventional hydraulic pressure supply system for a variable torque transfer as disclosed in the Japanese Patent Provisional Publication No. 2-68225. The four-wheel drive vehicle has an engine 1, a transmission 3a, a driving force transmission system 3, which system variably adjusts a distribution ratio of driving torque between front drive wheels 2FL and 2FR and rear drive wheels 2RL and 2RR, and a driving force distribution ratio control system 4. The conventional transfer clutch pressure supply system 5 included in the driving force distribution ratio control system is fluidly connected to a transfer clutch 7 such as a wet type multiple-disc clutch incorporated in a variable torque transfer 6 included in the driving force transmission system, to supply a controlled pressure to the transfer clutch. As shown in FIG. 2, the transfer clutch 7 usually includes a clutch drum 7a splined to the output shaft 17 of the transmission 3a, a plurality of friction plates 7b integrally connected to the clutch drum 7a, a clutch hub 7c rotatably supported on the outer periphery of the output shaft 17, a plurality of friction discs 7d integrally connected to the clutch hub 7c, a clutch piston 7e disposed in the right of the transfer clutch, and a cylinder chamber 7f defined between the clutch piston 7e and the clutch drum 7a. The inlet port 8 of the transfer 6 receives a controlled clutch pressure P.sub.C output from the hydraulic pressure supply system 5, with the result that the clutch piston 7e is pressurized to cause a relative displacement of the friction discs 7d to the friction plates 7b, and to force the friction discs 7d into frictional engagement with the friction plates 7b, and consequently to generate a desired engaging force of the clutch 7. In such a variable torque transfer 6, the front-and-rear wheel driving torque distribution ratio could be continuously controlled from a ratio of 0 : 100 to a ratio 50 : 50, depending on the magnitude of the controlled clutch pressure P.sub.C. As seen in FIG. 2, the conventional pressure supply system 5 has an oil pump 5c, which pump has a driven connection with an electrical motor 5a to pressurize oil stored in an oil reservoir 16, a one-way check valve 5d fluidly connected to the discharge port of the pump 5c for preventing back flow of the discharged oil back to the pump, a pressure accumulator 5e fluidly disposed in an oil supply line between the inlet/outlet port 8 and the check valve 5d, and a proportional electromagnetic type pressure control valve 5f disposed in the oil supply line downstream of the accumulator 5e. A secondary oil pressure output from the pressure control valve 5f, i.e., the controlled clutch pressure P.sub.C is determined usually on the basis of a current value of a command current I.sub.SOL output from a control unit 9 included in the driving force distribution ratio control system 4 to a proportional solenoid 5g of the pressure control valve 5f. The control unit 9 is electrically connected to a front-left wheel speed sensor 10FL, a front-right wheel speed sensor 10FR, and a rear wheel speed sensor 10R disposed in the vicinity of a propeller shaft 10 mechanically connected to the rear differential, so as to receive wheel speed data of the respective road wheels. That is, in case the above-noted proportional electromagnetic type pressure control valve is used to produce a controlled clutch pressure, the clutch pressure P.sub.C would be adjusted in proportion to the current value of the command current I.sub.SOL. The previously-noted front-and-rear wheel speed difference dependent traction control is effective to suppress acceleration-slip resulting from a change of vehicle driving state from a straight-ahead driving on high-friction road surface such as dry pavements to a rapid-acceleration driving or to a driving on low-friction road surface such as wet or icy roads. In case of a quick change of state from the greater-traction state to the less-traction state, a wheel speed detected by the rear-wheel speed sensor 10R becomes greater than a wheel speed detected by the respective front-wheel speed sensor 10FL and 10FR, owing to acceleration-slip experienced at the rear wheels 2RL and 2RR. In this case, the control unit 9 determines on the basis of the front-and-rear wheel speed difference that the acceleration-slip occurs at the rear wheels, and increasingly adjusts a value of command current I.sub.SOL to shift the transfer clutch 7 from a disengaged state to an engaged state by virtue of the hydraulic pressure supply system 5, thereby permitting a portion of driving torque to be applied to the rear wheels to be transmitted to the front wheels. In other words, when detecting acceleration-slip, the control unit 9 increasingly controls an engaging force of the transfer clutch 7 to shift from a two-wheel drive mode at which the driving torque can be transmitted from the transmission 3a only to the rear drive wheels 2RL and 2RR to a four-wheel drive mode at which the driving torque can be transmitted through the transfer 6 to the front wheels 2FL and 2FR as well as the rear wheels 2RL and 2RR. Thus, a driving stability could be enhanced by increasingly adjusting an engaging force of the transfer clutch.
On the other hand, the previously-noted four wheel anti-skid brake control is effective to suppress deceleration-slip experienced at rear wheels during quick braking. On quick braking, rear wheels tend to lock owing to a load shift from the rear road wheels to the front road wheels. To prevent the rear wheel lock, the control unit 9 increasingly controls the engaging force of the clutch 7 so as to permit a portion of braking torque to be applied to the rear wheels to be transmitted to the front wheels. Ordinarily, the control unit 9 determines the engaging force of the transfer clutch on the basis of an amount of engine braking, which amount is estimated by engine revolutions. In this manner, the braking balance between front and rear wheels, i.e., anti-skid characteristics would be remarkably improved.
However, in the total control according to which the variable torque control of the transfer clutch is performed in combination with the front-and-rear wheel speed difference dependent traction control, the four wheel anti-skid brake control or the like, when the controlled clutch pressure P.sub.C is gradually increased in response to the command from the control unit 9, there is a lag time until the transfer clutch 7 is shifted from its initial position wherein the friction plates 7b and the friction discs 7d are spaced apart from each other by a predetermined distance to its lightly engaging state wherein the friction discs 7d are brought into light contact with the friction plates 7b. The lag time is far from negligible. Owing to the lag time, the previously-noted prior art hydraulic pressure supply system 5 of a variable torque transfer would provide an insufficient responsiveness of the variable torque control of the transfer clutch.